As a rule, a variable speed engine operates against a mechanical load moment, which varies with a speed, which is determined in particular by a function of the speed of the engine. At present, such an engine generally features a comparator, which sets off a warning when the load moment of an engine exceeds a warning limit value. When the load moment of an engine for example increases quadratically with the speed, then at high speed correspondingly high load moments occur, according to which the warning limit value must be adjusted. As a consequence, in the case of a comparatively low speed of an engine, a considerable deviation from the load moment is often not detected because the load moment does not exceed the warning limit value that has been set for high speeds.